1. Technical Field
The present disclosure relates to a bone screw and, more particularly, to a semi-constrained bone screw and insertion instrument for use with a surgical implant.
2. Background of the Technology
The human spinal column is a highly complex structure. It includes twenty-four discrete bones, known as vertebrae, coupled sequentially to one another to house and protect critical elements of the nervous system. In between adjacent vertebrae is a disc. Each disc functions as a shock absorber, absorbing the impact of the body's activities and distributing pressure under compressive loads. Additionally, each disc forms a fibro-cartilaginous joint between adjacent vertebrae, allowing movement of the vertebrae and acting as a ligament to hold the vertebrae together.
For many reasons, such as aging and trauma, the discs may begin to deteriorate and weaken, potentially resulting in chronic pain, degenerative disc disease, or even tearing of the disc. Ultimately, the disc may deteriorate or weaken to the point of herniation, in which the inner portions of the disc protrude through the tear. A herniated disc may press against, or pinch, the spinal nerves, thereby causing radiating pain, numbness, tingling, and/or diminished strength or range of motion. The surgical removal of a diseased disc is a common procedure in current medical practice. Spinal fusion procedures involve removing a damaged disc and replacing it with an artificial disc, such as a bone graft or bone graft substitute. To keep the artificial disc and the vertebrae fixed in place while bone healing occurs, an external fixation device, such as a bone plate, is used. Bone plates are secured to the vertebrae using bone screws, which typically require drilling holes into the vertebrae.
A common problem associated with the use of such a bone plate or spinal prosthetic is the tendency of the bone screws to “back out” or pull away or otherwise withdraw from the bone into which they are mounted. This problem occurs, primarily, due to the normal torsional and bending motions of the body and spine. As the screws become loose and pull away or withdraw from the bone, the heads of the screws can rise above the surface of the bone plate or spinal prosthetic, which results in pain and discomfort for the patient or possibly the separation of the bone plate from one or more vertebrae.